In cooling apparatuses of a cooling water type which are used to cool automobile engines, thermostats have been used as control valves for regulating the quantity of cooling water circulating in the radiator, thereby making it possible to control the temperature of cooling water introduced in the engine. A control valve in such thermostats is inserted into part of a cooling water passage constituting the cooling apparatus and the temperature of cooling water is controlled to the required level by closing the control valve and causing the cooling water to circulate via a bypass passage, without passing through the radiator, when the cooling water temperature is low, and opening the control valve and causing the cooling water to circulate via the radiator when the cooling water temperature is raised.
Thermal elements using a thermally expandable body such as wax have generally been used as temperature sensors in such thermostats.
Conventional thermostats of this kind are known to have a variety of structures, primarily of a sleeve type (see, for example, Japanese Examined Utility Model Application No. S58-16003) and a diaphragm type (see, for example, Japanese Patent No. 3225386, pages 2–3, FIG. 1, FIG. 6).
The thermal element of a sleeve type which is described in the aforementioned Japanese Examined Utility Model Application No. S58-16003 has a structure in which a rubber sleeve is accommodated inside a metal container, the sealed chamber formed between the container and the rubber sleeve is filled with a wax serving as a thermally expandable body, and a piston as a movable member is slidably inserted via a compressible flowable body into the rubber sleeve. Further, in such a sleeve-type thermal element, the piston is actuated forward and backward by causing the piston to protrude by squeezing out the piston with the rubber sleeve provided around the piston.
On the other hand, the diaphragm-type thermal element described in the aforementioned Japanese Patent No. 3225386 has a structure in which a wax as a thermally expandable body is sealed at one end side of a case and volume changes of the wax are transferred by a diaphragm or flowable body to a piston serving as a movable body. In the thermal elements of a diaphragm type, if the wax expands, the diaphragm deforms so as to bulge in the direction from the wax, thereby applying pressure to the flowable body. As a result, this thermal expansion is converted into an axial movement and the piston serving as the movable body protrudes to the outside of the case.
The Japanese Patent No. 3225386 discloses as prior art technology a structure in which a rubber piston is used as a sealing material for preventing the flowable body from leaking to the piston side, the rubber piston provides for sealing, while sliding, and a back-up plate formed from a fluororesin is provided between the rubber piston and piston in order to prevent the rubber piston and piston from being fixedly attached to each other.
Further, Japanese Patent No. 3225386 also disclosed a structure in which the aforementioned rubber piston is omitted, the inward end of the piston borders on the inside of the flowable body, a ring-like sealing member is provided by inserting between the piston and case, and the flowable body is prevented from leaking to the outside.
Among the thermal elements having the above-described structure, the following problem was associated with the sleeve-type thermal element described in Japanese Examined Utility Model Application No. S58-16003. Thus, if the fluid which was the object of detection, such as cooling water, permeated into the container, the piston could not return into the container, and the so-called lift-up state was assumed. Further, when such a thermal element was used in a thermostat, the problem was that the warm-up performance of the engine could degrade because the valve remained open in the fluid channel.
Further, such sleeve-type thermal elements had a structure in which the piston was caused to protruded by squeezing out the sleeve. For this reason, a problem was associated with response. Another problem was that a long piston stroke could not be obtained. Furthermore, because normally the above-described sleeve repeatedly contracted and expanded, it could easily be fractured due to degradation, and once it has been fractured, the piston could not be caused to protrude. When such a thermal element was used in a thermostat, the valve remained in a closed state. Therefore, there was a risk of overheating, and special measures had to be taken to prevent it.
Furthermore, in the thermal element of a diaphragm type which represented prior art technology, by contrast with the above-described sleeve-type configuration, the wax that was a thermally expandable body was completely separated by the diaphragm. Therefore, no problem was associated with wax leakage. However, instead the problem was associated with the increased number of structural components because a diaphragm for separating the wax and transferring the expansion, a flowable body for pushing the piston instead of the conventional wax, and a rubber piston for sealing the flowable body had to be used, and the cost was difficult to reduce. Moreover, because wax was employed as a separate structure, a flowable body with flowability and lubricating properties better than those of the wax had to be used instead of the wax as a medium for moving the piston.
Further, in the above-described diaphragm-type thermal element, both the piston and the guide member holding the piston were made from a metal. Therefore, when a reaction force from another side was applied to the piston or guide member, the piston was tilted, the sliding resistance increased, and a problem was associated with the wear of the piston and guide member. Moreover, if the substances contained in the cooling water underwent chemical changes and were deposited as foreign matter on the sliding portions of the piston and guide member, the piston sometimes either moved slowly or could not move at all and maintenance was often required.
Further, the rubber piston that was a sealing member had a substantially round columnar shape and sealed, while sliding integrally with the piston. Therefore, the sliding resistance was large, resulting in poor response. Another problem was that in terms of endurance, the sealing performance was insufficient. Yet another problem was that because a diaphragm was used, the amplitude of this diaphragm was limited, thereby making it impossible to increase the operating stroke of the piston.
For this reason, as described in Japanese Patent No. 3225386, a structure was used in which the piston was caused to border on the inside of the flowable body and a sealing member was provided between the flowable body and guide member to prevent the flowable body from flowing out to the outside of the case and the cooling water from penetrating into the case. However, a large number of structural components such as the diaphragm and flowable body were still required and cost was difficult to reduce.
Further, in Japanese Patent No. 3225386, the aforementioned flowable body was sealed with a simple U-shaped packing between the piston and guide member, but the problem associated with this packing was that it created a risk of the flowable body leaking to the outside.
With the foregoing in view, it is an object of the present invention to obtain a thermal element with excellent response and endurance, in which the problems inherent to the above-described sleeve-type or diaphragm-type thermal elements are resolved, the cost can be reduced with a minimum necessary number of structural components, and the forward-backward movement of the piston with the prescribed stroke can be obtained by means of volume changes accompanying the expansion and contraction of a thermally expandable body.